You'd need to use the aggregation framework where you would run an aggregation pipeline that has a $group operator pipeline stage which aggregates the documents to create the desired counts using the accumulator operator $sum.
For the desired result, you would need to use a tenary operator like $cond to create the independent count fields since that will feed the number of documents to the $sum expression depending on the name value. The $cond operator can be used effectively to evaluate the counts based on the reply field value. It takes a logical condition as its first argument (if) and then returns the second argument where the evaluation is true (then) or the third argument where false (else). This converts the true/false boolean evaluated returns into 1 and 0 that will feed into $sum respectively:
"$cond": [ { "$eq": ["$reply", ">"] }, 1, 0 ]
So, if within the document being processed the "$reply" field has a ">" value, the $cond operator feeds the value 1 to the $sum else it sums a zero value.
Use the $project as your final pipeline step as it allows you to reshape each document in the stream, include, exclude or rename fields, inject computed fields, create sub-document fields, using mathematical expressions, dates, strings and/or logical (comparison, boolean, control) expressions. It is similar to SELECT in SQL.
The following pipeline should return the desired result:
Model.aggregate([ { "$group": { "_id": "$criterion", ">": { "$sum": { "$cond": [ { "$eq": [ "$reply", ">" ] }, 1, 0 ] } }, "<": { "$sum": { "$cond": [ { "$eq": [ "$reply", "<" ] }, 1, 0 ] } } } }, { "$project": { "_id": 0, "criterion": "$_id", "result.>": "$>", "result.<": "$<" } } ]).exec(function(err, result) { console.log(JSON.stringify(result, null, 4)); });
Sample Console Output
{ "criterion" : "story", "result" : { ">" : 1, "<" : 2 } }
Note: This approach takes into consideration the values for the $reply field are fixed and known hence it's not flexible where the values are dynamic and unknown.
For a more flexible alternative which executes much faster than the above, has better performance and also takes into consideration unknown values for the count fields, I would suggest running the pipeline as follows:
Model.aggregate([ { "$group": { "_id": { "criterion": "$criterion", "reply": "$reply" }, "count": { "$sum": 1 } } }, { "$group": { "_id": "$_id.criterion", "result": { "$push": { "reply": "$_id.reply", "count": "$count" } } } } ]).exec(function(err, result) { console.log(JSON.stringify(result, null, 4)); });
Sample Console Output
{ "_id" : "story", "result" : [ { "reply" : "<", "count" : 2 }, { "reply" : ">", "count" : 1 } ] }
